We will show preliminary results from a project devoted to determine the dependence of the Ly alpha luminosity function on redshift. We employ data kindly provided to us from the SHARDS survey. The data acquired in 25 relatively narrow band filters, covering the 150 arcmin2 GOODS-N field down to 26.5 magnitude in each filter, will be used by us for determining the population of of Lyman alpha sources from z= 3 to about z= 7, a big portion of the age of the universe. In the talk I will present the strategy we are following to detect the Lyman alpha sources. I will also present some preliminary results of the work done so far.